This invention is related to systems for space heating and/or cooling using Stirling cycle machines and particularly to improvements in the configuration, construction and operation of such devices.
Many cooling and heating systems presently used for temperature control of structures such as buildings or homes, and for cooling in motor vehicles principally rely on Freon (trademark for R-12 and R-22 refrigerants) as a working fluid. Although such systems operate in a reliable and efficient manner, their use is drawing increasing criticism since release of the refrigerant into the atmosphere during use or upon dismantling of the system has been shown to cause serious damage to the earth's ozone layer. The United States Government has enacted laws to eliminate or sharply curtail the use of chlorinated fluorocarbons (CFC) such as Freon and other gases harmful to the earth's ozone layer. Since destruction of the ozone layer has global implications, many of the industrialized countries of the world are cooperating in proposing and enacting laws to curtail the use of ozone damaging materials. In view of this situation, there is serious interest in providing thermal cycle systems for heating and cooling which use non-hazardous working mediums.
One type of thermal machine capable of providing space heating and cooling which can use non-polluting gases such as helium or hydrogen is the Stirling cycle machine. The Stirling cycle is a closed reversible thermodynamic cycle which can be implemented as a prime mover where heat is supplied and the output is in the form of mechanical power, as a refrigerator where mechanical power is supplied and the output is cooling capacity, or as a heat pump in which mechanical power is supplied and the output is in the form of heat (or in a reverse mode, cooling capacity). The assignee of this invention, Stirling Thermal Motors, Inc., is in the forefront of Stirling machine technology and has made numerous inventions in the art including those described by U.S. Pat. Nos. 4,579,046, 4,615,261, 4,669,736, and 4,707,990, which are incorporated herein by reference, and in current pending patent applications.
In accordance with conventional design practices for Stirling cycle devices, it was assumed that the Stirling cycle for so-called small temperature lifts (e.g., 10.degree. C.), such as for cooling and heating of spaces, was not suitable because of the low coefficient of performance (COP) provided by such a cycle. The reason for such low COP is that the adiabatic temperature fluctuations are large compared to the main temperature difference between the expansion heat exchanger and the compression heat exchanger of the device. The best method of obtaining high COP with a Stirling cycle would be to provide truly isothermal compression and expansion of the working medium. Attempts to provide isothermal compression and expansion with the Stirling cycle have not been successful to date. One approach to approaching isothermal compression or expansion is to reduce the pressure ratio of the machine (defined as the maximum divided by the minimum working fluid pressures in the compression space). However, by reducing the pressure ratio, the thermal output of the machine is also reduced. In view of these factors, Stirling cycle machines have not been viewed as good candidates to replace existing vapor compression heat pump and/or air conditioning systems.
In accordance with this invention, a Stirling cycle machine is provided which has an enhanced level of performance for space heating and cooling applications. The enhancements in performance are attributable in part to operating the device at low pressure ratio conditions where isothermal compression and expansion is approached. To compensate for the reduced thermal output of such a machine, it is charged with a working fluid at an unusually high mean pressure for this application. An excess so-called "dead volume" of the machine is intentionally incorporated for the purpose of decreasing its pressure ratio and increasing Coefficient of Performance (COP). The dead volume is optimally provided in the regenerator element of the Stirling machine since that element operates in a nearly isothermal fashion and putting it there results in lower friction losses when the machine is designed for low temperature lifts.
In accordance with a first embodiment of the present invention, a Stirling cycle heat pump/air conditioner is provided which is a "duplex" machine, having a Stirling cycle engine powered by a heat input such as by a direct gas flame which drives a Stirling cycle heat pump which provides a thermal output. The high mean pressure operation of the Stirling cycle heat pump/air conditioner operating at a relatively low pressure ratio provides the advantage that it can match the mean pressure used in the driving Stirling engine, thus allowing a common crankcase to be used. This embodiment features a Stirling engine substantially identical to those described in accordance with previously issued U. S. patents and currently pending applications assigned to Stirling Thermal Motors, with the addition of a piston for the Stirling heat pump/air conditioner coupled directly to the engine swashplate. For heating of a building during winter, the expansion heat exchanger absorbs heat from an outdoor heat exchanger coil and the compression heat exchanger rejects heat via an indoor heat exchanger coil. For summer air conditioning, valves could be used to reverse the heat exchangers which the expansion and compression space heat exchangers are connected to, causing indoor heat to be absorbed and rejected outside.
In a second embodiment of this invention, a Stirling cycle heat pump/air conditioner is driven by an electric motor enclosed within the pressure hull of the machine. This embodiment features the same enhancements in terms of reduced pressure ratio and excess dead volume placement. This device can be switched between summer cooling and winter heating modes in either of two manners. In one approach, the indoor and outdoor heat exchanging coils can be exchanged between the heat exchangers of the machine using valves or other circuit routing switches as in the first embodiment. Alternatively, the direction of rotation of the driving electric motor can be reversed which has the effect of changing the expansion heat exchanger to become the compression heat exchanger, and vice versa. This approach provides dual mode operation without complicated plumbing and valves.
The third embodiment of this invention is an open drive device principally adapted to provide air conditioning for motor vehicles. The device could be powered by a belt driven off the engine crankshaft. In addition to operating as an air conditioner, the unit is also capable of rapidly warming up the compartment of the vehicle even before the engine coolant becomes warm enough for compartment heating.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.